This R21 is directly responding to the PAR-12-197 on Improving Diet and Physical Activity Assessment. Research suggests that the environment influences lifestyle behaviors and contributes to racial and socioeconomic inequities in health, but overall, results are inconsistent and effect sizes are small. One potential explanation is measurement error in environmental exposures. Despite that many individuals spend considerable time outside their immediate neighborhood, most research has solely measured environmental exposures in a respondent's residential space and failed to take into account daily mobility. The objective of the proposed R21 is to examine the joint spatial and temporal stability of activity spaces (AS) derived from GPS data in a racially/ethnically and socioeconomically diverse sample of adults. Specific aims are: 1. Determine the optimal number and combination of days of GPS tracking needed to represent an individual's AS by testing the temporal stability within a person across days, types of days (e.g., weekend, weekdays), and between weeks. 2. Determine the sufficiency of using only one time period for GPS tracking by examining the temporal stability of an individual's AS within and between seasons. 3. As an exploratory aim, we will compare participant AS as derived from GPS tracking with those derived from questionnaires, with comparisons based on AS overlap, size, and environmental attributes. To achieve these aims, we will conduct GPS tracking with a socio-economically diverse sample of 144 African American, Mexican American, and white adults in Chicago for 30 days at baseline, with a second wave on a sub-sample six months later. We will measure physical activity using accelerometry and dietary intakes using multiple 24-hour dietary recalls. GIS will be used to construct measures of AS (e.g., route network buffer, kernel density estimation) and environmental attributes. This study will address key barriers to the advancement of a next generation of studies on the environment and lifestyle behaviors utilizing AS. Findings will supply essential information for a planned R01 on influences of AS environment attributes on lifestyle behaviors in a population-based multiethnic sample of adults.